I'm looking to implement an isolated (reinforced, >500V) isolation circuit on the PCBA side that would allow for SWD programming of MCU such as STM32 using JLINK / STLINK etc. The interface is quite fast in the range of several to tens of MHz.

There seems to be little or no IC's designed for this specific purpose, with the closest being I2C isolators such as ISO7731 (https://e2e.ti.com/support/isolation-group/isolation/f/isolation-forum/954510/iso7731-isolating-a-bidirectional-swd-connection) or larger solutions such as optoisolated adapter boards: https://www.segger.com/products/debug-probes/j-link/accessories/isolators/j-link-swd-isolator/.

Are you aware of any proven and robust implementation solving this problem?

Specifically, such a solution is most relevant in cases where it's beneficial to keep high voltage contained to the device and not create risks by using an isolated programmer which might not provide reinforced isolation and that requires for potentially poorly insulated cable to interconnect the parts.

  • 2
    \$\begingroup\$ You can buy an isolated ST-Link from ST. Why add anything to the board? \$\endgroup\$
    – Justme
    Apr 27, 2023 at 10:12
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    \$\begingroup\$ Usually, JTAG or SWD are not used in normal operation, only during debug & programming. There is not enough demand for such an IC, especially since isolated programmers exist. \$\endgroup\$
    – Lior Bilia
    Apr 27, 2023 at 10:12
  • \$\begingroup\$ That's a fair point, but that means that the cable going from device to the programmer could be dangerous if damaged or if there are other problems. In my case I would like to increase safety by containing the isolated areas on distant parts of the PCBA. \$\endgroup\$ Apr 27, 2023 at 10:23
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    \$\begingroup\$ For example, isolated STLink has an insulation rating of 2.5 kV on the insulation PCBA, but will the cable going to the programmed board have this rating? Most likely not. \$\endgroup\$ Apr 27, 2023 at 13:49
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    \$\begingroup\$ Can you power the device from the programmer? That way you could program it while it's powered off and safe. \$\endgroup\$
    – Drew
    Apr 28, 2023 at 17:56

1 Answer 1


You could use an off the shelf USB isolator with power although the user would have to not touch the programmer and you would need rated insulation.

A better way would be to use a digital isolator, if you wanted the unit powered during flash or debug, a power isolator would also be needed.

This is a nice isolator that can run at 1 MHz which I believe is fast enough for the four wire swd interface.


A better thing would be to isolate all the digital between the high voltage section and the microprocessor.

  • \$\begingroup\$ SWD is a bi-directional line and will need an I2C or equivalent isolator. \$\endgroup\$
    – Jeroen3
    May 2, 2023 at 14:29
  • \$\begingroup\$ You are right the last time I did this it was for the JTAG, I have updated \$\endgroup\$
    – Voltage Spike
    May 2, 2023 at 15:37

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